Several types of device are known for use in air for obtaining high absorption coefficients:
Porous materials having interstices in which the sound energy is transformed into heat by turbulance and friction within said interstices: when the incident wave arrives at the solid rigid portion of the wall it has lost its energy and reflection is very low.
Elastic panels constituting a spring-and-mass system: when the vibration period of this device is of the same order as that of the sound waves, a portion of the incident energy is transformed into mechanical energy and is then dissipated by deformation or internal friction.
Cavity resonators which also act as a spring-and-mass system in which the mass and the resilience belong to air: at resonance, a portion of the energy is dissipated by air headloss in the neck of the resonator.
In all these cases, the difficulty is obtaining effectiveness over a sufficiently wide frequency range:
The first type is effective only at high frequencies.
The drawback of the last two types is having their effectiveness limited to very narrow frequency bands centered on the natural frequencies of said systems.
In water, the same problem also arises: i.e. the problem of increasing the absorption coefficient of a wall:
This may be required either for the purpose of reducing the energy reflected from an underwater wall which is located in the proximity both of noise sources and of sound detectors. Such a wall may be a part, for example, of a dynamically positioned offshore drilling platform or oil exploration platform.
Another purpose may be to simulate wave propagation in an infinite medium within a noise-measuring laboratory.
An ideal covering would both:
prevent any reflection, even partial reflection, of the incident waves; and
operate over a wide frequency band, and in particular, in water, over a band which includes low frequencies in the range 10 Hz to 1000 Hz.
Some prior coverings are quite inadequate when the ambient medium is constituted by a liquid and, in particular, at low frequencies below 1000 Hz.
That is why French patent document FR-A No. 2 562 699 and the corresponding documents EP-A No. 0161458 and U.S. Pat. No. 4,560,028 (Alsthom Atlantique F.degree. 13666) propose a covering having the following features, some of which, at least for some of their functions, are common between this prior covering and a covering in accordance with the present invention.
This prior covering has a rear face for applying to a rigid base wall and a front face for being immersed in an ambient medium, in particular a liquid, through which sound waves are propagating. It comprises:
auxiliary walls fixed perpendicularly to the base wall and leaving elongate energy-dissipating ducts extending along a direction which is also perpendicular to said wall, said ducts being filled with a dissipating substance having a dynamic viscosity which is greater than that of the surrounding liquid so that the substance dissipates the energy by friction when it oscillates longitudinally in said ducts under the action of said sound waves; and
a clearance space behind said dissipation ducts in order to allow the dissipating substance to perform such oscillations freely.
In this prior covering, said dissipating substance is a mixture of liquids and said clearance space is occupied by flexible enclosures inflated with a gas whose elasticity makes possible and conditions the oscillations of said liquid. The liquid must be separated from the ambient fluid by a membrane which must be both transparent to the sound waves to be damped and sufficiently strong to avoid being damaged in operation. Providing such a membrane can sometimes be problematical.
The object of the present invention is to provide effective absorption of sound waves in a liquid ambient medium, even when at low frequencies, by means of a covering which is more robust and easy to manufacture.